Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

to a very bright light source, which flushes almost all of the metastable electrons totheir ground state, emptying most of the F-centers.CR systems operate with a workflow pattern very similar to screen-film radiography.CR cassettes are exposed just like screen-film cassettes, and they are thenbrought to a reader unit, just as film-screen cassettes are brought to the film processor.The similarity in handling of CR and screen-film cassettes contributed to theinitial success of CR. One of the advantages that CR over screen-film radiographyis its much larger dynamic range (Fig. 11-4); in other words, the exposure latitudewith CR is much wider than with screen-film systems. Consequently, retakes dueto overexposure or underexposure are rare with CR. Because of the difficulty ofobtaining precise exposures in the portable radiographic setting, where phototimingusually is not available, CR is often used for portable examinations. Although CRis capable of producing images with proper gray scale at high and low exposurelevels, quality assurance efforts should still be in place to ensure proper exposure levels.CR images exposed to low exposure levels, while maintaining proper gray scalein the image, have higher levels of x-ray quantum noise ("radiographic mottle").CR images produced at high exposures have low quantum noise but result inhigher x-ray dose to the patient.Charged-coupled devices (CCDs) form images from visible light. CCD detectorsare used in most modern videocameras and in digital cameras. The principal featureof CCD detectors is that the CCD chip itself is an integrated circuit made of crystallinesilicon, similar to the central processing unit of a computer. A CCD chip hasdiscrete pixel electronics etched into its surface; for example, a 2.5 X 2.5 cm CCDchip may have 1024 X 1024 or 2048 X 2048 pixels on its surface. Larger chips spanning8 X 8 cm have been produced. The silicon surface of a CCD chip is photosensitive-asvisible light falls on each pixel, electrons are liberated and build up inthe pixel. More electrons are produced in pixels that receive greater light intensity.The electrons are kept in each pixel because there are electronic barriers (voltage) oneach side of the pixel during exposure.Once the CCD chip has been exposed, the electronic charge that resides ineach pixel is read out. The readout process is akin to a bucket brigade (Fig. 11-5).Along one column of the CCD chip, the electronic charge is shifted pixel by pixelby appropriate control of voltage levels at the boundaries of each pixel. The chargefrom each pixel in the entire column is shifted simultaneously, in parallel. For linearCCD detectors, the charge packet that is at the very bottom of the linear arrayspills onto a transistor, where it produces an electronic signal that is digitized. Theentire line of pixels is thus read out one by one, in a shift-and-read, shift-and-readprocess. For a two-dimensional CCD detector, the charges on each column areshifted onto the bottom row of pixels, that entire row is read out horizontally, andthen the next charges from all columns are shifted down one pixel, and so on.CCD cameras produce high-quality images from visible light exposure and arecommonly used in medical imaging for fluoroscopy and digital cineradiography. Inthese devices, the amplified light generated by the image intensifier is focused withthe use of lenses or fiberoptics onto the CCD chip (Fig. 11-6). For small field-ofviewapplications such as dental radiography (e.g., 25 X 50 mm detector), an inten-